Tunable single-mode lasers are essential elements in high information rate optical telecommunication systems. The single mode feature avoids pulse dispersion in long spans and high data rate applications. The tunability feature facilitates assembling a series of channels in a densely packed wavelength division multiplexed (WDM) configuration.
The traditional tunable semiconductor lasers such as the distributed feedback (DFB) laser and distributed Bragg reflector (DBR) laser can, at best, produce optical frequency shifts which are directly proportional to the induced change in the refractive index in the semiconductor. This frequency shift is too limited for a wavelength division multiplex multichannel optical communication system which is typically characterized by 20 or more channels with a frequency separation of 20 to 100 GHz. This invention is directed toward an optical filter which can be located inside the cavity of a semiconductor laser to provide a tunable laser which has a wide tuning range and can be used with a WDM multichannel optical communication system.